Photoelectric tube



March 17, 1959 G. BERGSON PHOTOELECTRIC TUBE Filed July 23. 1954 INVE N TOR. Eu S'TAV BEBE'SUN 11 T'IOR NE Y United States Patent This invention relates to photo-sensitive detectors, and more particularly to photoelectric cells of the type having photoemissive cathodes.

The primary object of this invention is'. to provide a photoelectric tube which may be energized by radiant energy passing completely through the tube. To accomplish this object, the photoelectric tube cathode is formed as a semi-transparent surface through which the radiant energy may pass.

Another object of this invention is to provide a photosensitive tube which is characterized by its simplicity of construction and its economy of manufacture;

A still further object of this invention-is to provide a photoelectric tube which is responsive to predetermined wavelengths within the ultra violet range.

A further object of this invention is to provide a photoelectric tube for use in a system wherein radiant energy is passed through successive tubes, each tube being reh of radiant energy passed therethrough. r t

The novel features that are considered characteristic of this invention are set forth with more particularity sponsive to different components of the composite spectra in the appended claims. The invention itself, howeverf both as to its organization and method of operation, as "well as additional objects and advantages thereof, will best be understood from'the following description when read in connection with the accompanying drawings, in

" which: a t t Figure 1a is a perspective view of a photoelectric tube constructed in accordance withthe invention;

t Figure lb is a perspective view of a photoelectric tube similar to that shown in Figure 1a, but having a modified anode structure; and

Figure 2 is a diagrammatic longitudinal sectional view of a gas analyzing apparatus including a schematic circuit diagram of an electrical indicating circuit, illustrating one form of equipment with which the photoelectric tubes of the invention may be used. t

- Referring now to the drawings and particularly to Figure 1a, the photoelectric tube 10 of the invention is provided with-an envelope 11. A rod-shaped anode 12 projects through the side of the envelope 11 to provide "an external connection point' and ispreferably of any.

material which lends itself satisfactorily to a glass to metal seal. A semi-transparent cathode 13' of suitable 1 photo-sensitive material is deposited on the inner rear window of the envelope 11 and is provided with an external terminal connection point 14 which has a con-,

ductive connection through the wall of the envelope 1 1. The term photo-sensitive cathode, as used herein, refers "to a cathode which is sensitive to wavelengths in at least a portion of the spectra of radiant energy including infra-red, ultra violet, and visible wavelengths. The

' open end of the envelope is sealed by a front window 15 which renders the tube airtight so as. to maintainla a vacuum. The front window and envelopeare made of a material which does. not unreasonably attenuate. wavel; leng hs tths radi nt en reywhic t de ired to detec .matter of design.

The tube is :generally cylindrical in shape so as to admit the radiant energy which is to energize the tube through the front window 15, and allow the energy to pass through the cathode 13 and outof the tube to a second tube or the like, while atthe same time energizing the cathode 13.

One method of forming the cathode on the inner surface of the tube is by evaporation of the cathode material to be deposited. In this process, the cathode material is placed on a filament structure whichis then placed :near the surface to be coated and then heated to boil off the material. The process is continued until the desired degree of opacity is reached whereupon the evaporation is stopped. If desired, the filament structure may be left in the envelope to provide ananode element for the tube.

In one embodiment of the invention a platinum cathode is formed by coating a tungstenfilament with platinum and evaporating the platinum onto the surface of the envelope of the tube Those portions of the envelope which should not have a metallic coating such as those adjacent the anode element, are masked. After the evaporation process the masking material may be removed to provide a clean surface not having leakage paths formed by the evaporated material. The evaporation of the platinum onto the envelope is halted after the desired degree of transparency of the cathode was reached. The process may be carried out ina vacuum to prevent contamination of the platinum since the platinum itself without further activation is to serve as the cathode material of the tube. Since this tube as described is designed for use in the ultra violet region ofthe spectrum,

the envelope and the front window are constructed of good quality quartz or some other material which is easily permeable by wavelengths in the-ultra violet 35- t region. r

The cathode material for a photoelectric tube constructed in accordance with the invention primarily a For the detection of radiant energy having wavelengths in the ultra violet region, the platinum cathode described above would be sensitive to wavelengths shorter than 196 millimicrons. That is, electrons would be released from the platinurn' cathode by wavelengths of the ultraviolet spectrum shorter than 196 millimicrons, which electrons would be attracted to the anode of thetube. A cathode of tungsten would be responsive to wavelengths of substantially 260 millimicrons or shorter. The wavelengths at which various metals become responsive is a property of the Work function of the metal, and the formulas for computing the response of various metals together with the physical characteristics of the metals are set forth in the text Photoelectric Phenomena by Hughes and Du Bridge, first edition 1932.

' If desired, the cathode of the phototubes may be made sensitive to the visible spectra or to the infra-red spectra by sensitizing with a caesium or some other sensitizing material. The sensitizing material may be enclosed in a pill and placed within the envelope of the tube, and

' after the tube is evacuated and baked, the sensitizing -material is released from the pill and deposited on the cathode to render it photo-sensitive. If after the application of the sensitizing material, the cathode becomes .too opaque, heat may be applied to the cathode to drive off the excess caesium.

-In Figure lb, a modification of the photoelectric tube 10 of Figure 1a is shown in which the anode structure is modified. An annular anode 12b is provided as part of the envelope structure. The anode should be of a material which lends itself to the making of a glass to metal seal.v Ifdesired, the anode could be located entirely within the envelope 11b, however, an external connection to such an anode would be. required. 1

It is not intended that the invention be limited to a particular process of constructing the photoelectric tube or to particular cathode materials therefor, the above being described by way of example to enable a clear understanding of this invention.

Referring to Figure 2, which shows a gas analyzing or testing equipment illustrating one form of apparatus in which the photoelectric tubes of the invention may be used, a sample of the gas tobe tested is admitted into an absorption chamber 20 through a pair of inlet and outlet pipes 22. If desired, thegasllnder testmay be continuously passed through the absorption chamber 20. The absorption chamber 20 has a cylindrical body portion with opposite ends thereof closed by the windows 25 and 26. The windows 25 and 26 are tightly sealed with ap- .propriate gaskets to the cylindrical body portion by any suitable fastening device such as a pair of screw-on caps 23 and 24.

-A source of radiations 21, which by way of example, may be a source of ultra violet radiations such, as a G4S1l lamp, which is connected with a pair ofterminals 27 for supplying an energizing potential, is positioned adjacent to the window 25 for directing radiant energyinto the absorption chamber 20. The windows 25 and 26 are made of a material such as quartz or other material which is easily permeable by the radiant energy-from the source 21 The beam emerging from the window 26 is directed successively through a series of photoelectric tubes. 29, 31 and 33 constructed in accordance with the invention, which have semi-transparent cathodes or cathodes which'allowa portion-of the beam, to pass through and responsive to a wide range of ultra violet frequencies. A first optical filter. 28 is interposed between the absorption chamber 20 and the photoelectric tube 29. A second optical filter element 30 is interposed between the first photoelectric tube 29 and. the second.photoelectric'tube 31, and a third optical filter 32 is positioned between the second. photoelectric tube 31 and the third photoelectric tube 33.

In describing the operation of the gas analyzing system, it will be assumed that the gas under test contains two components which it is desired to measure. One of the components will be assumed, by way of example, to have a strong absorptive spectra at wavelengths around 185 millimicrons, and the other to have a strong absorptive spectra around 254 millimicrons. The beam emanating from the source 21 has a relatively wide band of ultra violet wavelengths including the wavelengths at 185 and 254 millimicrons. The beam is passed through the'ab- 'sorption chamber 20 which contains the gas sample having the two components mentioned above, and wavelengths at 254 and. 1,85 millimicrons are'at least partially absorbedby these components. The optical --filter 28 attenuates wavelengths below 185 millimicrons so that any minor absorptive spectras of the components below 185 millimicrons will not affectthe energization of, the cathodes of the photoelectric tubes-in the system. The filter 28 is optional equipment and is not necessary to the effective operation of the. gas analyzing system.

It may be assumed that the photoelectric tubes [29, 31

and. 33-are of'simil'ar construction, and are responsive generally to all the wavelengths of interest in the ultra violet range. The cathode of the photoelectric tube 29is therefore responsive to, the wavelengths at both 185 and 254 millimicrons and will give a relative indication of the radiant energy at 185 and 254 millimicrons which is incident thereon as wellas miscellaneous changes inabsorption. The filter 30 attenuates the wavelengths of the radiant: energy of 185 millimicrons or less, so that the second photoelectric tube 31 will only be affected by the relative absorption at '254 millimicrons as' wellas incidental changes. in absorption by the windows in the absorption chamber 20, the. filters 28' and-30,- andthe windows ofthephotoelectriotubes; The third photoelectric tube 33, which is a comparison or standard cell, is connected in circuit with the second photoelectric tube to provide a differential indication of the relative absorption of the particular component at 25 4 millimicrons are sensitive. The filter 32, which is located between the photoelectric tubes 32 and 33, attenuates the wavelengths equal or less than 254 millimicrons so that the response of the cathode of the third photoelectric tube is essentially outside the range of any of the wavelengths absorbed by the components of the gas under test and serves as a standard comparison photocell whose current measures the changes in lamp intensity, window absorption of the'absorption cell and the phototubes and filters, and the like.

The system could be continued upward in wavelengths for several cells so that several components contained in the gas mixture under test could be detected and measured. It is recognized that the filters could be incorporated as the front surface of the phototube thus eliminating an extra part. If desired, phototubes having cathodes which respond only at certain wavelengths could be used as well, as-will be hereinafter described.

The photoelectric tubes 29, 31 and 33 are each provided with an anode element and a cathode element. The anodesofthe photoelectric tubes 29, 31, and 33 are'connected with a terminal 35 which supplies a polarizing potential from a power source, not shown. A second power supply'terminal 36 is connected with a point of fixed referencepotential or ground.

The cathodes 37, 38 and 39 of the photoelectric'tubes 29,31 and 33, respectively, are connected with the electrical indicating circuit. The cathode 37 is connected with a control electrode 41 of an amplifier tube 40 which also has-'an anode 42 and a cathode 43. A resistor 44 is connected between the control electrode 41 and ground to provide a direct current grid return circuit for the tube 40, andialso'tocompletethe direct current path ofthe photoelectric tube 29. A variable cathode bias resistor 45 is connected between the cathode 43 and ground to .provide an operating bias potential for the amplifier tube cludes, together with the grid 51, an anode 52 and a cathode 53. The cathode 53 is connected through a variable cathode bias resistor 54 to ground while the a anode is connected through a resistor 56 and part of the variable resistor 47 to the terminal 35 which supplies the .polarizing potential for the tube.

A meter 57 is connected between the anodes 42 and 52 to indicate the differential. anode potential of the tubes 40 and 50, which is afunction of the absorption of the wavelengths at millimicrons as will hereinafter be explained.

As. mentioned above, the cathode 58 of the photoelectric tube 31 is connected with the control electrode 61 of the amplifier tube 60. The amplifier tube 60 also has ananode 61 and a cathode 63. -A variable cathodev bias ,resistor 65.is connected between the cathode 63 and ground. The anode 62 is connected through an anode loading resistor 66 and a portion of a variable resistor 67 to the terminal 35 which supplies the polarizing potential for the amplifier tubes. i

The cathode 39 of the photoelectric tube 33 is connected with a control electrode 71 of the amplifier tube 70. A resistor 74 provides a grid return for the tube 70 'and'also serves to complete the direct current path of the photoelectric tube 33 to ground. The tube 70' has a cathode 73 which is connected to ground through 'a variable bias resistor 75, and also has an anode 72 which is-connectedthrough an anode loading resistor '76-and a .portion ofthe variable resistor 67 to the polarizing potential terminal 35. A meterr77 is connected between the anodes 62 and 72 to provide means for indicating the differential anode potential which is a function of the absorption of therwavelengths at 254 millimicrons in the present example.

To adjust the system for operation, the tubes40 and 50 should have equal anode potentials when none of the components of the gas to be detected are present, or in other words, when none of the radiant energy is absorbed. This may be done by evacuating the absorption chamber 20 or supplying a reference gas therein, and adjusting the tap 47a of the resistor 47 to increase the load resistance in the anode circuit of one of the tubes and reduce it in the anode circuit of the other tube. A sample of gas containing a component which absorbs ultra violet energy at 254 millimicrons and no componentswhichabsorb 185 millimicrons is then admitted to the absorption chamber and the cathode resistors 45 and 55 are adjusted so that the tubes 40 and 50 will operate on a predetermined portion of their operational curves whereby they will have equal response at the anodes thereof to the input signal from the tubes 29 and 31.

If a gas is introduced into the absorption chamber 20, which contains components that absorb wavelengths of 185 and 254 millimicrons respectively, the energization of the photoelectric tube 29 will be changed as a function of the absorption of the radiant energy by both components, in addition to the changes in absorption of radiant energy due to the filters, phototube windows, absorption chamber windows, etc. Since the filter 30 absorbs 185 millimicrons, the photoelectric tube 31 is responsive to changes in absorption of the radiant energy by the component which absorbs 254 millimicrons in addition to the other changes in absorption due to the filters, phototube windows, and absorption chamber windows, etc.

Thus the signals fed to the control electrodes 41 and 51, tubes 40 and 50 by the photoelectric tubes 29 and 31, are equal except to the extent of the changes in energization of the phototube 29 due to absorption of the 185 millimicrons wavelengths. Accordingly, this differential input voltage on the grids is amplified and appears between the anodes 42 and 52 of the tubes 40 and 50. The potential difierence between the anodes 42 and 52, which is a function of the quantity of gas present which absorbs wavelengths at 185 millimicrons, is indicated on the meter 57.

In like manner, the reading on the meter 77 gives an indication of the relative quantity of the component of gas present in the sample which absorbs 254 millimicrons. The photoelectric tube 31 is responsive to change in absorption due to the presence of a component which absorbs 254 millimicrons in addition to the other miscellaneous absorptions. Since the filter 32 stops the 254 millimicrons, the energization of the phototube 33 is only due to the miscellaneous changes in absorption of the radiant energy. The outputs of the photoelectric tubes 31 and 33 are fed to the tubes 60 and 70 respectively, the only difference between the two being due to the absorption of the components of the gas at 254 millimicrons. The differential input voltage is amplified and appears between the anodes 62 and 72 which difference in potential is indicated by the meter 77. The readings of the meter 77 is thus a function of the absorption 'of radiant energy at 254 millimicrons and gives a relative indication of the quantity of a component of gas present in the absorption chamber 20 which has an absorption spectra at 254 millimicrons.

In another embodiment of the gas analyzing equipment, phototubes may be used which have cathodes that are sensitive at different predetermined wavelengths. For example, as described hereinbefore, certain metals are responsive to different wavelengths, hence by fabricating the cathodes of the photoelectric tubes with the. proper materials, the system described above can function without the use of filter elements. Platinum is sensitive to wavelengths of about 196 millimicrons and shorter, Tungsten, on the other hand, is responsive towavelengths of about 260 millimicrons and shorter. To incorporate photoelectric tubes into the system shown in Figure 1, the tube 29 would have a cathode of tungsten, thus being responsive to changes in the absorption at both and 254 millimicrons, while the photoelectric tube 33 would have a cathode of platinum and would be responsive to only 185 millimicrons. The standard cell 33 would preferably be fabricated of a material responsive to a relatively wider range of ultra violet wavelengths, and would be provided with a filter to remove the 185 and 254 millimicron components. The phototubes could be connected in the electrical measuring circuit in the same manner as described above in connection with-Figure 2. i i i In accordance with the foregoing description, an improved photoelectric tube of simplified construction has been provided which has a semi-transparent cathode which enables the tube to be energized by a beam of radiant energy passing therethrough.

What is claimed is:

l. A photoelectric tube including a tubular body member with a pair of transparent windows afiixed to opposite ends thereof to form an air tight envelope and to provide an unobstructed path for radiant energy successively through said windows, an. anode supported by said envelope, a semi-transparent photosensitive cathode coated on one of said windows, and terminal connection means for said cathode and anode accessible exteriorly of said envelope.

2. A photoelectric tube as defined in claim 1 wherein the semi-transparent photo-sensitive cathode element comprises a thin film of platinum.

3. A photoelectric tube for use in a gas analyzing system of the type wherein radiant energy is passed successively through the gas and a plurality of photoelectric tubes to measure the relative absorption by the gas of the various spectral components passed therethrough comprising in combination, an envelope having a tubular body member and first and second transparent windows positioned generally in parallel planes transverse to the axis of said tubular member at opposite ends thereof so that a path for radiant energy is provided successively through said first and second windows, said first window comprising a radiation permeable window, means providing an anode in said envelope, external circuit connection means for said anode extending through a Wall of said envelope, means providing a semi-transparent coating of a photosensitive material on the inner surface of said second window, said semitransparent coating being responsive to radiant energy passing along the axis of said envelope to release electrons, and external circuit connection means conductively connected with said semi-transparent coating of photosensitive material extending through the wall of said envelope.

4. A photoelectric tube responsive to predetermined wave lengths of radiant energy and adapted to pass radiant energy therethrough to a further utilization device comprising in combination a cylindrical envelope having one end thereof closed by an integral transparent spherical section, a transparent circular disc secured to the free end of said envelope to provide an air tight cover therefor, and to provide a radiation admitting window for said photoelectric tube, means providing an anode electrode within said envelope having external circuit connection means extending through the wall of said envelope, means providing a semi-transparent coating of a photosensitive material on the inner surface of said spherical section, said semi-transparent coating being responsive to radiant energy passing along the axis of 7 said envelope to' releaseelectrons, and external circuit connection means conductively connected with said semitransparentcoating of photosensitive material extending through the wall of said envelope.

5-. A photoelectric tube for use" in a ,gas analyzing system of the-type wherein radiant energy is passed successively through thega's and a plurality of photoelectric tubes each adapted" to be responsive to different bands ofwave' lengths of'saidradiant energyto' measure the relative absorption by the gas of the various spectral components passed thei'ethrough comprising in combination, a-cylindrical envelope having a first and second oppositely disposed transparentwindow positioned generally ih parallel planes transverse to the axis of said cylindrical envelope, said first window' comprising an optieal filter for eliminating certain of said wave lengths of's'a'id radiant energy incident thereon, means providing an anode: in said envelope; external circuit connection means for said anode extending through the wall of References Cited in the file of this patent UNITED STATES PATENTS 2,056,392 De Boer et a1. Oct. 6, 1936 2,073,522 Kingdom-ct a1. Mar. 9," 1937 2,297,492 Michaelai i n 222 Sept. 29,- 1942 2,444,915 Cade' July 13, 1948 2,613,330 Bruining etal. Y- -Oct. 7, 1952 2,676,282 Polkosky Apr. 20,1954 

